it sounds like it is the expansion vessal, whats the pressure like on the boiler when its cold? and does it go to 3mb when on for a few mins and leek out the over flow pipe out the back?is the boiler making any hissing sounds if so the heat exchanger is on the way out!or the pipe going to the vessel could be blocked down to dirt in the system, have seen it on all makes of boilers?
peter dublin
November 2010